1. Technical Field
The invention relates to a variable-power optical system and an imaging device that are suitably used for a small-sized device having an imaging function, such as a digital still camera, a mobile phone having a camera built therein and a personal digital assistance (PDA).
2. Description of the Related Art
In imaging devices such as a digital still camera, decrease in size of the entire device has been required along with decrease in size of imaging elements such as CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor). With the decrease in size, decrease in thickness is required for an imaging lens, particularly, a variable-power optical system (zoom lens) to reduce its total length. Japanese Patent No. 3196283 describes a variable-power optical system that is used for a digital still camera and that has a five-group configuration. The variable-power optical system described in Japanese Patent No. 3196283 is a straight-type optical system in which optical members constituting the lens system are arranged in a straight line without a direction of the optical axis being changed. Here, the size of the imaging device in a thickness direction is determined substantially by a length from the most-object-side optical member to an imaging element. On the other hand, in order to meat the recent requirement for increase in pixels and enhancement in performance of the imaging element, the number of lenses increases. Thus, it becomes difficult to decrease the total length of the lens system. Accordingly, it is difficult to decrease the total thickness of the imaging device. As a result, in order to decrease the thickness of the imaging device, a so-called bending-type optical system in which an optical path of the lens system is bent in the middle way has been developed.
In the bending-type optical system, a reflecting member such as a right-angle prism is disposed in a first lens group to bend the optical path in its middle way by about 90°, thereby decreasing the length of the optical system in the thickness direction. As such a bending-type variable-power optical system, one that has a four-group configuration and that moves a second lens group and a fourth lens group at the time of varying the magnification has been known. With the requirement for a higher variable-magnification ratio, one which employs a bending-type variable-power optical system and which has five groups as a whole has been developed, thereby accomplishing a higher variable-magnification ratio than the four-group configuration (see JP 2006-301543 A (corresponding to US 2006/0238886 A) and JP 2006-98686 A (US 2006/0066955 A)). In the variable-power optical system described in JP 2006-323051 A, a second lens group and a fourth lens group are moved at the time of varying the magnification. In the variable-power optical systems described in JP 2006-301543 A and JP 2006-98686 A, a fifth lens group is also moved at the time of varying the magnification in addition to the second lens group and the fourth lens group. In the variable-power optical system described in JP 2006-301543 A, the fifth lens group has a focusing function and the focusing function is performed from the infinity to a vicinity point by moving the fifth lens group toward an image formation surface. At the time of varying the magnification, a focal length is changed by the linear motions of the second lens group and the fourth lens group. Variation of the image formation surface is corrected by the nonlinear motion of the fifth lens group.
However, in the optical system described in Japanese Patent No. 3196283, the focal length f1 of the first lens group is large and thus, the total length of the lenses is also large. Therefore, such a configuration is disadvantageous in decrease in size. In the optical system described in JP 2006-301543 A, the fifth lens group is moved toward the image formation surface to perform the focusing function. Accordingly, when the fifth lens group is moved at the time of focusing, a distance of an exit pupil is greatly varied and change due to shading easily occurs. Since the fifth lens group gets close to the image formation surface at the time of focusing, dust or scratches attached to the lens surfaces of the fifth lens group can affect the image quality. In the optical system described in JP 2006-323051 A, when it is assumed that the first lens group is divided into a front group and a rear group at a reflecting surface, the focal length f1r of the rear group of the first lens group is set to be greater, thereby increasing the total length of lenses. Accordingly, such a configuration is disadvantageous in decrease in size. In the optical system described in JP 2006-98686 A, since the focal length f2 of the second lens group is great, the total length of the lenses is great. Thus, such a configuration is disadvantageous in decrease in size.